(1) Field of the Invention
The present invention relates to a developing device that develops an electrostatic latent image formed on a photoreceptor, and to an image forming apparatus including the developing device.
(2) Description of the Related Art
An image forming apparatus that employs electrophotographic method is generally provided with a developing device that develops, using toner, an electrostatic latent image formed on a photoreceptor drum. The toner image formed on the photoreceptor drum by the developing device is transferred to, and subsequently thermally fixed on a recording sheet.
The developing device includes a cylindrical developing sleeve located opposite and in parallel with the photoreceptor drum, and a cylindrical magnetic body fitted in the developing sleeve so as to be coaxial. The magnetic body is fixed so as not to rotate. The magnetic body is provided with magnetic poles at a plurality of positions along the circumferential direction thereof. The magnetic poles each extend along an axial direction (i.e., a direction along the axis). The developing sleeve rotates about the magnetic body in a predetermined direction.
With respect to the above-mentioned developing device, the developing sleeve rotates to transport magnetic toner stored in the developing device to a position opposite the photoreceptor drum, where the electrostatic latent image on the photoreceptor drum is developed with the toner. In this way, a toner image is formed on the photoreceptor drum.
Each end portion of the developing sleeve in the axial direction is a portion in which the magnetic body does not exist (hereinafter, referred to as a “no-magnet portion”). Toner is not transported to the no-magnet portions of the developing sleeve. Accordingly, toner image is not formed on portions of the photoreceptor drum which are opposite the no-magnet portions. Note that toner image is not formed on a portion on each end portion of the photoreceptor drum. Hereinafter, the portion is referred to as a “non-image-formation portion”.
Some image forming apparatuses of this type are constituted to allow for transferring a maximum-size toner image formed on the photoreceptor drum onto a recording sheet having a size larger than the maximum size, so as to form an image on the recording sheet without a margin.
For example, when the maximum-size a toner image formed on the photoreceptor drum by the developing device is A3 size, the toner image on the photoreceptor drum is transferred to and fixed onto a recording sheet having a size larger than A3 size. The size larger than A3 size is generally referred to as “enlarged A3 size”.
Marks indicating the positions of the corners of a recording sheet of A3 size are formed using toner on the photoreceptor drum simultaneously with a toner image of A3 size. These marks are generally referred to as “printer's marks”. The toner image formed on the photoreceptor drum is transferred to and fixed on a recording sheet of enlarged A3 size with a margin formed along the periphery of the recording sheet.
The recording sheet of enlarged A3 size on which a toner image has been fixed is cut according to the “printer's marks”. Thus, the peripheral margin of the recording sheet is removed, and the size of the recording sheet is reduced to a predetermined size (i.e., A3 size). Note that, hereinafter, the printing of a predetermined size on which a toner image is formed without margin along the periphery of the recording sheet as described above is referred to as a “marginless printing”.
As described above, when a toner image of A3 size is formed on the photoreceptor drum, toner may be transported on a portion of the developing sleeve that borders the no-magnet portion. The magnetic body does not exist in the no-magnet portion. Therefore, smaller amount of magnetic force acts on the toner on the boundary portion adjacent to the no-magnet portion of the developing sleeve. Consequently, there is a risk that a portion of the toner on the boundary portion under the effect of the centrifugal force generated by the rotation of the developing sleeve is scattered to the no-magnet portion on which magnetic force does not act, and sticks to the non-image-formation portion of the photoreceptor drum.
The toner sticking to the non-image-formation portion is not transferred to a recording sheet of A3 size, but may be transferred to the peripheral margin of a recording sheet of enlarged A3 size. The peripheral margin of a recording sheet of enlarged A3 size having a toner image formed thereon is cut off after the recording sheet is printed out, and a “marginless printing” of A3 size is obtained. However, if a user happens to see the toner sticking to the margin of the recording sheet before the margin is cut off, the user may have a concern that some problem has occurred in the image forming apparatus. Therefore, it is preferable that toner is prevented from sticking to the margin of a recording sheet of enlarged A3 size.
A developing device generally includes seal members so as to prevent leakage of toner to the outside of the developing device from the ends of a developing sleeve in the axial direction thereof. Each of the seal members is pressed against, and in surface contact with, either one of end portions of a photoreceptor drum. Some proposals have been made to prevent toner from sticking to the non-image-formation portion of the photoreceptor drum by using the seal member as described above.
According to these proposals, each of the seal members is pressed against either one of end portions of a photoreceptor drum, and shields a non-image-formation portion of the photoreceptor drum from toner. The non-image-formation portion is the portion that faces the no-magnet portion of the developing sleeve. One of end surfaces of each of the seal members that is on the inner side of the developing device (i.e., closer to the center of the developing sleeve in the axial direction) is located in the vicinity of either one of the ends of the magnetic body in the axial direction, and is in contact with the non-image-formation portion of the photoreceptor drum.
According to the above-described structure, even when a portion of the toner transported on the developing sleeve is scattered between the non-magnetic portions of the developing sleeve and the non-image-formation portions of the photoreceptor drum, the scattered toner sticks to the tip surfaces of the seal members and portions of the surfaces of the seal member that are opposite the non-magnetic portion. Accordingly, there is no risk that toner sticks to the non-image-formation portions of the photoreceptor drum that are covered by the seal members.
However, the tip surfaces of the seal members are in contact with the non-image-formation portions of the photoreceptor drum. Therefore, toner sticking to the tip surfaces of the seal members may be brought into contact with the non-image-formation portions of the photoreceptor drum. There is a risk that the toner brought into contact with the non-image-formation portions may subsequently stick to the non-image-formation portions in a stripe pattern as a result of the rotation of the photoreceptor drum, and the toner is transferred to a recording sheet of enlarged A3 size simultaneously with a toner image.
When the toner is transferred to the peripheral margin of the recording sheet of enlarged A3 size in a stripe pattern, it is highly probable that a user recognizes the stripe pattern, and the user may have a concern that something is wrong.